Thermal barrier and method of use

ABSTRACT

A thermal barrier fabricated from pumice, or pumice-like material, and a suitable binder is provided. It may be formed as a self-supporting or load-bearing structural member, or as a thick coating for IR signature reduction. A first embodiment may be used to build a structure, e.g., a room onboard a ship that serves as an effective thermal barrier. Should a fire start in the room, the thermal barrier prevents rapid spreading of the fire and provides crews additional time to fight the fire. A second embodiment, as a thick coating, reduces the IR signature of a radiating body, such as an exhaust stack, by a factor of four. This thick coating helps shield an object from IR surveillance devices or seekers, resulting in much shorter acquisition and tracking times for these IR devices and seekers. In addition to benefits as a thermal barrier, structural members using concepts disclosed for this invention may provide inherent blast and shock resistance.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The invention described herein may be manufactured and used by or forthe government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

FIELD OF THE INVENTION

The present invention pertains to a robust thermal barrier suitable foruse as a structural member or as a coating. In particular, a firstconfiguration may be a self-supporting or load-bearing wall of astructure, e.g., a room in a ship. A second configuration may be asurface coating for energy conservation or reducing the infrared (IR)signature of an object.

BACKGROUND

Today's navies are operating with reduced crews that are being asked torespond to emergencies with the same efficiency as the larger crews ofthe past. This impacts the ability to protect resources, especiallygiven that some of the crew has been replaced with additional highexplosive ordnance, increasing the volume susceptible to hostile actionor catastrophic accidents. Additionally, new and retrofit shipbuildingis being scrutinized for implementation of cost saving initiatives, toinclude impact on life cycle costs. A solution that reduces operationalrisk as well as capital investment and maintenance expense is needed. Inthe recent past, pumice has been used as part of a technologicalsolution to enhance the U.S. Navy's mission readiness by providing aneffective barrier against sympathetic detonation of weapons stored inmagazines and transport containers. The natural characteristics of apumice-based barrier include both shock absorption and thermalinsulation as discussed in more detail below. Of course, othermaterials, natural or man-made, with properties similar to pumice may besubstituted.

Pumice used for construction often is mixed with Portland cement, water,and other additives to provide desirable attributes of weatherproofing,appearance, and water and wear resistance. See U.S. Pat. No. 5,759,260,Method for Using Lightweight Concrete for Producing a CombinationTherefrom and a Combination Produced Thereby, issued to Groh, Jun. 2,1998. A common use of lightweight materials, such as pumice, is forproduction of pre-formed panels or other structures. See U.S. Pat. No.5,440,846 (panel with insulated core), Construction for Building Panelsand Other Building Components, issued to Record, Aug. 15, 1995; U.S.Pat. No. 4,567,705 (panel for fire protection), Fire ProtectionArrangement and Method for Positioning Same, issued to Carlson, Feb. 4,1986; and U.S. Pat. No. 4,259,824 (panel with some inherent insulativeproperty), Precast Concrete Modular Building Panel, issued to Lopez,Apr. 7, 1981. Another use of lightweight materials is for smallerbuilding components such as construction blocks. See U.S. Pat. No.4,641,470, Construction Element, issued to Baumberger, Feb. 10, 1987 inwhich a block of lightweight materials, having cavities cast therein,has the cavities filled, in a second step, with insulating materials.

Applying commonly used construction materials in formulation of thematerial's mixture and the resultant structure assures localavailability and an inherent confidence in the product since the builderis familiar with the performance of known materials. See U.S. Pat. No.5,860,268, Light-Weight Concrete Door, issued to McWilliams, Jan. 19,1999 in which a metal frame, wire mesh, hinges, and wooden molds, allcommon construction material, are combined with a concrete mix and anovel air entrainment admixture. . See U.S. Pat. No. 5,875,607, Low-CostExterior Insulation Process and Structure, issued to Vohra, Mar. 2,1999, in which bags of insulating material that may contain pumice aspart of the mix, are placed against existing exterior walls, connectedto the wall, covered with stucco wire, and stuccoed for a finishedsurface.

Should one wish to particularly exploit a particular characteristic ofmaterial having the properties of pumice as used in a building material,one needs to carefully select a binder, and method of application of thebinder, in order to optimize that characteristic. One such desirablecharacteristic of a pumice-like material is its resistance to conductingheat, in particular, high heat.

When exploiting a number of desirable characteristics of pumice,however, no one characteristic is likely to be optimized. See U.S. Pat.No. 4,231,884, Water Retardant Insulation Composition Comprising TreatedLow Density Granular Mineral Material and Finely Divided Limestone orSimilar Carbonate or Silicate Mineral Particles and Method for UsingSame, issued to Dorius, Nov. 4, 1980. The '884 patent provides aninsulative composition that is also a water retardant, a corrosionpreventative, and capable of use in building a load-bearing wall. Toaccomplish all of these objectives, certain additional coatings areprovided for the lightweight inorganic material. As well, the physicalcomposition of the mixture is adjusted to accommodate each objective. Noone objective is being optimized in the mixture.

Pumice has been used for the fabrication of refractory materials. SeeU.S. Pat. No. 5,228,914), Pumice Containing Composition, issued toMiceli, Jul. 20, 1993 (a mixture of crushed pumice, calcium aluminate,glass fibers and water for use in ovens, heaters, and otherhigh-temperature applications). For this application, the precise makeupof the mixture must be followed to attain the refractory material, animportant ingredient being calcium aluminate.

Pumice, in combination with a binder of cement, such as PORTLAND cement,and water, and other optional ingredients such as volcanic ash, scoria,vermiculite, mineral wool and even kerosene, was used to create aninsulative thermal barrier. See U.S. Pat. No. 4,803,107, Light WeightThermal Insulation Material Product and Process, issued to Knowles, Feb.7, 1989. Again, the product resultant from the above process wasintended to address a number of objectives such as water retardation,the “R-factor” of the product for use in residences, and structuralstrength. Further, pumice comprised less than twenty percent of themixture so that objectives other that fire retardation could beaddressed by the product.

Aggregates of inorganics have also been an ingredient in coatings thatmay be applied by spraying, brushing, rolling, troweling, or usinggenerally accepted stuccoing methods. See U.S. Pat. No. 5,556,578,Aggregate Containing Hydration Water in Spray Applied Fireproofing,issued to Bemeburg et al, Sep. 17, 1996 and U.S. Pat. No. 5,034,160,Sprayable Fireproofing Composition, issued to Kindt et al, Jul. 23,1991. The '578 patent describes a slurry for spraying on structuralcomponents, such as steel beams, to provide a flame-retardant surface.The slurry comprises a cementitious binder, such as PORTLAND cement andwater, and a hard aggregate having hydration water, such as bauxite,together with optional additives, such as shredded polystyrene aggregateand starches, to aid application. The '160 patent describes amulti-element composition suitable for use as a sprayed-on coating. Thecomposition includes a cementitious binder such as PORTLAND cement andwater, a porous aggregate that could include pumice, a fibrous material,an air-entraining agent, and a rheopectic (a fluid mixture that, whensubjected to a shear force, increases in viscosity) agent. Both the '578patent and the '160 patent are directed to a solution of the problem ofpumping the mixture over large distances, e.g., the upper floors ofhigh-rise buildings and, as such, are addressing a number of competingobjectives.

Optimizing the use of pumice-like material for construction may key onthe attributes of strength, cost, appearance, and ease of applicationwith little or no attention paid to thermal conduction. A carefullycrafted mixture, optimized for performance as a thermal barrier, may notmeet one or more of the above requirements for general construction. Infact, this “carefully crafted” mixture may have been considered andsubsequently rejected because it did not meet the builder's moreimmediate objectives.

As noted above, pumice has been used as an ingredient in constructionmaterials where a lightweight substitute for concrete or adobe has beencalled out in specifications. It has also been used for packing aroundexplosives to preclude sympathetic detonation of a neighboring explosiveshould one of a package be detonated. See U.S. Pat. No. 5,158,173,Weapons Storage Container to Prevent Sympathetic Detonation of AdjacentWeapons, issued to Halsey et al, Oct. 27, 1992, incorporated herein byreference in particular to the shock and blast resistance of pumicebarriers, and U.S. Pat. No. 5,160,468, Method for Preparing a StorageContainer for Explosive Rounds, issued to Halsey et al, Nov. 3, 1992,incorporated herein by reference in particular to the shock and blastresistance of pumice barriers. Further, there exist any number ofmethods to provide a separate insulation barrier, examples of which arethe '607 patent and the '884 patent.

Combining pumice-like material with a suitable binder for pouring intoforms for making, in a single process, a thermal barrier with structuralintegrity has not been perfected prior to this invention.

SUMMARY OF THE INVENTION

A preferred embodiment of the present invention comprises a mixture ofpumice or pumice-like particles and a binder, whereupon said mixture iseither poured into a form, with optional reinforcement, for fabricatinga structure or applied as a coating. The binder can be any of a numberof suitable admixtures, including, but not limited to, a Portlandcement; a plaster; an epoxy; a resin, including a polyester or epoxyresin; and a polymer binder, or a combination of the above.

In one preferred embodiment, the pumice or pumice-like particles are ofa size between ¼″ (0.63 cm) and ⅜″ (0.94 cm) and the epoxy is acommercially available two-part epoxy. This mixture is poured into formsmuch like concrete forms but not requiring the same strength since thematerial is much lighter than concrete. It can be poured into formspositioned horizontally, vertically, or any angle in between. It canalso be poured into forms enclosing non-traditional shapes, such asogives, truncated pyramids, cylinders, and irregular shapes to encloseodd-shaped devices such as gun turrets or boilers with accompanyingductwork and piping. Because of the relatively low density of thematerial, lightweight removable partitions or panels can be formed fromit, facilitating modification or maintenance work.

In a second preferred embodiment, a mixture of pumice or pumice-likeparticles and binder is used to coat heat emitting devices in order toconserve heat or reduce the infrared signature of the device, or both.This coating may serve as a replacement for asbestos barriers nowsubject to strict environmental regulations.

Although the same size particles may be used in the coating mixture asfor forming a structural member, ease of application may call for theuse of smaller particles on the order of ⅛″ or smaller. However, it isbeneficial to maintain a minimum particle size. The insulation value ofthis thermal barrier depends on formation of air spaces, or voids,between the particles. Since the particles do not pack tightly, onesurface mating exactly with another, these voids are an inherentconsequence of forming such a coating. This coating may be applied toany surface, including those of irregular conformation and texture.Optionally, a first adhesive layer may be needed to insure properadhesion of the mixture to the surface. Material to coat such anadhesive layer is selected based on the materials content andconformation of the surface to be coated.

Advantages of preferred embodiments of the present invention, include:

dual use fabrication as a structural member and a thermal barrier;

high weight percent of naturally heat resistant material;

lightweight, high strength substitute for concrete;

able to be formed into any shape;

increased fire resistance, lowering operating costs and risks;

reduced fire insurance rates;

simplified design of alternate configurations;

inexpensive fabrication;

reduced system complexity requiring a single member to serve multiplefunctions;

environmentally friendly, using natural materials and suitable for usewith

the most environmentally compliant binders;

able to be sprayed, brushed, rolled, troweled, or applied as a stucco toany surface;

impervious to sporadic water damage;

reduced system capital costs;

increased operational readiness;

low maintenance costs;

increased flexibility;

high reliability;

particularly suitable for renovations and modifications; and

readily applied as a coating to existing structures and objects.

Embodiments of the present invention can be applied to land, sea, orairborne vehicles or fixed facilities. Incorporating the invention intoa design saves capital as well as operations and maintenance costs.Further, a preferred embodiment of the present invention may be usedanywhere in the world that common building practices are followed.

Preferred embodiments are fully disclosed below, albeit without placinglimitations thereon.

BRIEF DESCRIPTION OF DRAWINGS

1. FIGS. 1A and 1B offer a comparison of a preferred embodiment of thepresent invention to an aggregate composition used in commonconstruction applications.

2. FIG. 2 depicts a preferred embodiment of the present invention as acoating over a typical flue for a two-element stack.

3. FIG. 3 is a graph of flame test results for a preferred embodiment ofthe present invention.

4. FIG. 4A depicts locations of internal thermocouples used in a flametest of a container protected by a preferred embodiment of the presentinvention.

5. FIG. 4B depicts locations of external thermocouples used in a flametest of a container protected by a preferred embodiment of the presentinvention.

6. FIG. 5 is a graph of flame test results from the thermocouplesdepicted in FIGS. 4A and 4B.

7. FIG. 6 depicts a representation of an IR signature of an objectcoated with a preferred embodiment of the present invention.

DETAILED DESCRIPTION

Refer to FIG. 1. FIG. 1A depicts a top view and projected side view of asolid cylinder 110 as fabricated for a preferred embodiment of thepresent invention. The pumice or pumice-like particles indicated by thedarkened areas 112 represent thinly-coated particles as joined by abinder used in a preferred embodiment of the present invention. Thelight areas 113 represent voids in the structure. These voids are airgaps. The projected side view shows the irregular surface 111 resultingfrom molding or casting a mixture of a preferred embodiment of thepresent invention into a solid cylinder, the cylinder being boundtogether by the thin coating of binder applied to each particle, a bondoccurring only at the location each coated particle physically touchesanother particle. Compare this to FIG. 1B (Prior Art).

FIG. 1B shows a solid cylinder 120, representative of prior art, similarin size and shape to the solid cylinder of FIG. 1A. The solid cylinder120 also is comprised of aggregate particles 123 and a binder 122. Thebinder 122 completely fills the voids between the aggregate particles123 such that there are no voids (air gaps) like the voids 113 of FIG.1A. Further, the “finished” surface 121 of the cylinder, as shown inboth a top view and a side view is smooth (when viewed from a macroaspect) as compared to the solid cylinder 110 and its irregular surface111 of FIG. 1A.

It is this provision for voids or air gaps, that provides a preferredembodiment of the present invention with properties that result in anexcellent thermal barrier that also has sufficient structural strengthfor use as self-supporting or load-bearing structural members, with orwithout added reinforcement elements. Because a mixture of a preferredembodiment of the present invention can be applied as a coating, it ispossible to retrofit existing systems or even use it as a supplement toconventional construction methods. For the initial coating to properlyadhere to certain surfaces, additional adhesives or agents may have tobe applied to the surface to be coated with a preferred embodiment ofthe present invention's mixture, or incorporated in the mixture for thefirst coating.

In one embodiment of the present invention, the binder is selected tothinly coat pumice or pumice-like material crushed to a uniform size ofabout ¼″-⅜″ (0.635 cm -0.95 cm) in the longest dimension, for subsequentplacement in a form suitable for fabricating a structural member. Thisthin coating may be applied in a thickness from about 0.004″ (0.1mm)-0.04″ (1.0 mm). The form may also have placed therein reinforcingmaterial such as a steel mesh, “rebar,” or any number of polymercompounds suitable as a reinforcing agent. After curing, the mixtureforms a “unitized” structural member of a thermal barrier.

Refer to FIG. 2. FIG. 2 shows a thick coating 201 of a preferredembodiment of the present invention enveloping a conventional two-partflue 202. This coating utilizes pumice or pumice-like particles of auniform size, perhaps of a smaller size than what would be used for astructural member, e.g., ⅛″-¼″ (0.32 cm -0.635 cm), and perhaps adifferent binder with more solvents to assure an even flow, e.g., apolymeric resin. To summarize, this second embodiment employs a highlyviscous mixture for coating existing surfaces, such as exhaust stacks,doors, and portable generators for the military.

In general, a thermal barrier using a preferred embodiment of thepresent invention is fabricated using a porous aggregate comprisinguniform pumice or pumice-like particles of ¼″-⅜″ and a two-partslow-curing epoxy. The epoxy and aggregate are mixed, e.g., by tumbling,resulting in a thin epoxy coating on each aggregate particle. Themixture is poured into a form or casting and sets up as a porous solid.Upon setting, only the contacting surfaces of the individual aggregateparticles are joined, leaving considerable voids (air gaps) in theproduct, yielding a very porous structural member.

This particular fabrication method yields benefits in addition to theefficient production of a characteristically porous structural member.First, the amount of epoxy required to just adhere adjoining surfaces ofaggregate particles is considerably less than that required to producean impervious or non-porous solid by filling the voids with binder. Thisreduces fabrication costs considerably. Second, the voids within thestructural member make for a much lighter member, an especiallyimportant design consideration when installation on a vehicle orwaterborne vessel is the goal. Third, the voids enhance the insulativeproperty of the structural member, trapped air being a major contributorthereto. Fourth, the porous material is better able to withstand theeffects of shock and blast, as compared to a structural member withsmaller or no voids. Note that the very benefits for which thisembodiment is being touted may well make it unsuitable for use in anenvironment where water resistance or wear may be a controlling factor.

Although a two-part epoxy has been used in the above example, otherbinders are suitable given that a thin complete coating of the aggregateparticles can be effected without a subsequent filling of the voids(pores). The final environment in which the barrier will be used willdictate both the type of binder and any additional agents or structuralreinforcement needed.

A test of a sample block, fabricated as a thermal barrier from atwo-part epoxy and ¼″-⅜″ pumice, was conducted. Thermocouples wereembedded at several depths within a block approximately 12″×4″×4″. Fourthermocouples were placed at depths of 1″, 2″, 3″, and on the other sideof the block (4″) from the flame applied by an oxyacetylene cuttingtorch. Results are shown in FIG. 3 as D1, 1″; D2, 2″; D3, 3″; and D4, 4″displacement 300. The heat from the torch peaked 301 at about 1925° F.(1050° C.) at the 1″ depth D1 but at the 3″ depth D3 the temperatureleveled off 302 at about 550° F. (290° C.) after three minutes while atthe 4″ depth D4 the temperature quickly stabilized 303 at 330° F. (185°C.) after about one minute and remained at that level for the next twominutes of the test.

Additional testing confirmed that an embodiment of the present inventionworks as a thermal barrier for a container protected by an embodiment ofthe present invention. A canister was placed on a grill much like abarbecue grill available at a public picnic grounds. A fire wasinitiated under the grill and allowed to burn out. Thermocouples wereplaced within the canister at various locations, from very near thesource of the flames at the bottom of the canister to the very centerand top of the canister. See FIG. 4A for a representation of a top viewof the test setup and FIG. 4B for identification and placement of thethermocouples from a side view, the surface comprising a preferredembodiment of the present invention as a thermal barrier for acontainer. Test results are presented in FIG. 5. Thermocouple TC25 401placed on the bottom of the grill rack 402 started at about 275° F.(120° C.), peaked at about 1500° F. (815° C.), and was at about 560° F.(295° C.) at the conclusion of the test. Thermocouples TC1-TC5 403 hadsimilar measurements to TC25 401, as they were placed within the thermalbarrier itself at each of the bottom four corners and within the centerof the barrier's bottom layer. Thermocouples TC6-TC10 404, placed oneinch inside the canister at its bottom on the four corners and in themiddle, exhibited the greatest heating inside the canister, peaking atabout 430° F. (220° C.) for TC8 404 and at only about 200° F. (95° C.)for TC6 404. Of the remaining thermocouples, only TC's -12, -13, and -14405 approached a peak temperature of 300° F. (150° C.), being somewhatfurther away from the canister's bottom than TC's -6--10 404.Thermocouples TC16-24 406, placed on the sides of the outside of thecontainer, but within the barrier exhibited the least amount of heatingon average, with peaks and starting temperatures staying at or belowabout 210° F. (100° C.). This indicates that the pumice barrier on thebottom of the canister was sufficient to prevent heating of the canisteroutside's side surfaces displaced even a short distance from the bottomthat was subjected to direct flames.

A third test was conducted of a preferred embodiment of the presentinvention used as a coating for purposes of IR shielding. A 12″×12″×1″slab representing a thick pumice or pumice-like aggregate coating of apreferred embodiment of the present invention was subjected to heatingon one side. The slab was fabricated in the same manner as the12″×12″×4″ block above. FIG. 6 is a representation of an IR signature ofthe plate taken after heating the slab on one side to about 360° C. Thedark splotches in the top right quadrant 601 indicate “hot spots” thattranslate to a peak temperature of about 90° C. This demonstrates that athick coating of an aggregate mixture containing a pumice or pumice-likematerial, as in a preferred embodiment of the present invention, reducesIR signature by a factor of four.

The above descriptions should not be construed as limiting the scope ofthe invention but as mere illustrations of preferred embodiments. Forexample, although examples discussed above pertain mainly toconstruction, the use of a preferred embodiment of the present inventionin other applications, such as refractory devices, commercial ovens, andthe like, is not precluded. The scope shall be determined by appendedclaims as interpreted in light of the above specification.

We claim:
 1. A thermal barrier composition, comprising: particles ofpumice and of a size within a uniform range; and a binder for joiningsaid particles; wherein said binder, thinly covering said particles,joins said particles for use as the thermal barrier, and wherein saidparticles comprise from about 85-99.5 weight percent of the barrier, andsaid binder comprises from about fifteen to one-half weight percent ofthe barrier.
 2. The barrier of claim 1 wherein said binder is spread ina thickness of from about 0.1-1.0 mm.
 3. The barrier of claim 1 whereinsaid barrier is a structural member.
 4. The barrier of claim 3 whereinsaid structural member is selected from the group consisting of a wall,a door, a floor, a ceiling, and a roof.
 5. The barrier of claim 1incorporating a reinforcing material.
 6. The barrier of claim 1 whereinsaid barrier is a coating.
 7. The barrier of claim 6 wherein saidcoating is a coating of about 0.125 cm-3.5 cm thickness, wherein saidcoating may be applied in multiple layers.
 8. The barrier of claim 6wherein said coating may be applied by any of the methods selected fromthe group consisting of: spraying, rolling, brushing, troweling, andstuccoing.
 9. The barrier of claim 6 wherein additional adhesives oragents are incorporated in said barrier to facilitate the applicationand durability of said coating.
 10. The barrier of claim 1 wherein saidbinder is selected from the group consisting of: an epoxy, a two-partepoxy, a plaster, a cementitious binder, a polymeric binder, and aresin.
 11. The barrier of claim 1 wherein said binder is a two-partepoxy.
 12. The barrier of claim 1 wherein said particles are sized fromabout 0.125 cm-3.5 cm in the longest dimension of said particles. 13.The barrier of claim 1 wherein said particles are sized from about 0.5cm-1.0 cm in the longest dimension of said particles.
 14. An enclosedstructure, having structural components comprising: particles of pumiceof a uniform range of sizes; and a binder for joining said particles;wherein said binder, spread thinly over said particles, joins saidparticles as a mixture for use as a thermal barrier, and wherein saidparticles comprise from about 85 to 99.5 weight percent of said mixture,and said binder comprises from about fifteen to one-half weight percentof said mixture.
 15. The structure of claim 14 wherein said binder isspread in a thickness of from about 0.1-1.0 mm.
 16. The enclosedstructure of claim 14 wherein said binder is selected from the groupconsisting of: an epoxy, a two-part epoxy, a plaster, a cementitiousbinder, a polymeric binder, and a resin.
 17. The enclosed structure ofclaim 14 wherein said binder is a two-part epoxy.
 18. The enclosedstructure of claim 14 wherein said particles having a uniform range ofsizes are sized from about 0.125 cm-3.5 cm in the longest dimension ofsaid particles.
 19. The enclosed structure of claim 14 wherein saidparticles having a uniform range of sizes are sized from about 0.50 cm-1.0 cm in the longest dimension of said particles.
 20. The enclosedstructure of claim 14 wherein said structural components include areinforcing material.